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Re: [PATCH v3 1/2] tools/xenstore: set oom score for xenstore daemon on Linux

On 30.07.21 15:26, Ian Jackson wrote:
Juergen Gross writes ("[PATCH v3 1/2] tools/xenstore: set oom score for xenstore 
daemon on Linux"):
Xenstored is absolutely mandatory for a Xen host and it can't be
restarted, so being killed by OOM-killer in case of memory shortage is
to be avoided.

Set /proc/$pid/oom_score_adj (if available) per default to -500 (this
translates to 50% of dom0 memory size) in order to allow xenstored to
use large amounts of memory without being killed.
+## Type: integer
+## Default: 50
+# Percentage of dom0 memory size the xenstore daemon can use before the
+# OOM killer is allowed to kill it.
  ## Type: string
  ## Default: @LIBEXEC@/boot/xenstore-stubdom.gz

Thanks for working on this.  I approve of the principle.

I have one question about detail:

+       rm -f @XEN_RUN_DIR@/xenstored.pid
+       XS_PID=`cat @XEN_RUN_DIR@/xenstored.pid`
+       echo $XS_OOM_SCORE >/proc/$XS_PID/oom_score_adj

The effect of all this is that the value specified in
XENSTORED_OOM_MEM_THRESHOLD is transformed before being echoed into
/proc, by being multiplied by -10.


Of course an alternative would be to ask the user to specify the
tuneable directly but given its rather more obscure semantics I think
it is reasonable to have this done by the script.

Correct. Otherwise the user would need to know about the oom_score_adj

But maybe we could add something to the doc comment ?

   # (The specified value is multiplied by -10 and echoed into
   # /proc/PID/oom_score_adj.)


Why? This is an internal implementation detail. I don't see why the
user needs to know how this is accomplished. What is unclear with the
XENSTORED_OOM_MEM_THRESHOLD semantics as described?

There is no other parameter with an explanation how it's semantics are
being accomplished.


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